1. Field of the Invention
The present invention relates to transmission blocking vaccines against malaria and methods of preventing the transmission of the disease. The invention further relates to recombinant virus encoding a Plasmodium falciparum sexual stage surface protein and to host cells infected therewith.
2. Background Information
Malaria continues to exact a heavy toll from mankind. The major cause of malaria in humans is the parasite Plasmodium falciparum. Approximately 25 percent of all deaths of children in rural Africa between the ages of one to four years are caused by malaria.
The value of various vaccines to combat malaria is appreciated through an understanding of the life cycle of the parasite. Infection in man begins when young malarial parasites or xe2x80x9csporozoitesxe2x80x9d are injected into the bloodstream of a human by the mosquito. After injection the parasite localizes in liver cells. After approximately one week the parasites or xe2x80x9cmerozoitesxe2x80x9d are released into the bloodstream. The entry of the parasites into the bloodstream begins the xe2x80x9cerythrocyticxe2x80x9d phase. Each parasite enters the red blood cell in order to grow and develop. When the merozoite matures in the red blood cell, it is known as a trophozoite and schizont. A schizont is the stage when nuclear division occurs to form individual merozoites which are released to invade other red cells. After several schizogonic cycles, some parasites, instead of becoming schizonts through asexual reproduction, develop into large uninucleate parasites. These parasites undergo sexual development.
Sexual development of the malaria parasites involves the female or xe2x80x9cmacrogametocytexe2x80x9d and the male parasite or xe2x80x9cmicrogametocyte.xe2x80x9d These gametocytes do not undergo any further development in man. Upon ingestion of the gametocytes into the mosquito, the complicated sexual cycle begins in the midgut of the mosquito. The red blood cells disintegrate in the midgut of the mosquito after 10 to 20 minutes. The microgametocyte continues to develop through exflagellation and releases 8 highly flagellated microgametes. Fertilization occurs with the fusion of the microgamete into a macrogamete. The fertilized parasite is known as a zygote that develops into an xe2x80x9cookinete.xe2x80x9d The ookinete penetrates the midgut wall of the mosquito and transforms into the oocyst within which many small sporozoites form. When the oocyst ruptures the sporozoites migrate to the salivary gland of the mosquito via the hemolymph. Once in the saliva of the mosquito, the parasite can be injected into a host.
Malaria vaccines are being developed against different stages in the parasite""s life-cycle which includes the sporozoite, asexual erythrocyte, and sexual stage. Each development increases the opportunity to control malaria in the many diverse settings within which the disease occurs. Sporozoite vaccines would prevent mosquito-induced infections. First generation vaccines of this type have been tested in humans. Asexual erythrocytic stage vaccines would be useful in reducing the severity of the disease. Multiple candidate antigens have been cloned and tested in animals and in humans.
One type of vaccine being investigated to slow or reverse the worsening epidemic of malaria is a transmission blocking vaccine [Miller et al., Science 234:1349 (1988)]. Transmission of Plasmodium falciparum from host to mosquito vector can be blocked by monoclonal antibodies against a 25 kDa sexual stage surface protein, Pfs25, expressed on zygotes and ookinetes [Vermeulen et al., J. Exp. Med. 162:1460 (1985)]. The gene encoding Pfs25 has been cloned [Kaslow et al., Nature 333:74 (1988)], and the deduced amino acid sequence revealed a striking feature, the presence of four tandem epidermal growth factor (EGF)-like domains. EGF-like domains are cysteine rich and depend on proper disulfide bond formation for structural integrity [Savage et al., J. Biol. Chem. 247:7612 (1972)]. It is not surprising, therefore, that of the monoclonal antibodies known to block transmission, none recognize the reduced Pfs25 antigen [Vermeulen et al., J. Exp. Med. 162:1460 (1985) and Carter et al., Prog. Allergy 41:193 (1988)], suggesting that for at least some of the blocking epitopes, disulfide bonds are involved in creating proper conformation.
A subunit vaccine for controlling endemic malaria in developing countries needs to induce high, long-lasting antibody titers, and be produced in large amounts, at the lowest possible cost. Bacteria or yeast provide a simple means of recombinant protein expression that is inexpensive, if the recombinant products are easily purified and immunologically effective. Live attenuated viruses, such as vaccinia or adenovirus, are an attractive alternative because they too are inexpensive to produce, and in addition, are easily transported and administered. Furthermore, as the antigen is produced in the mammalian host""s cells, proper folding and post-translational modification are more likely to occur than in prokaryotic expression systems.
Accordingly, it is one object of the present invention to provide a transmission blocking vaccine for humans against malaria which is inexpensive to produce and easily transported and administered.
It is a further object of the present invention to provide a means of producing transmission blocking antibodies against malaria in humans.
Various other objects and advantages of the present invention will become apparent from the following description of the invention together with the figures.
In one embodiment, the present invention relates to a recombinant virus encoding a 25 kDa sexual stage surface protein, Pfs25 of Plasmodium falciparum or encoding at least 6 amino acids of Pfs25 and to host cells infected therewith which express the Pfs25 protein on their surface.
In another embodiment, the present invention relates to a transmission blocking vaccine against malaria. The vaccine can comprising a recombinant virus encoding Pfs25 of Plasmodium falciparum, a recombinant virus encoding at least 6 amino acids of Pfs25 or Pfs25 purified from a recombinant system such as host cells of the present invention, in an amount sufficient to induce immunization against malaria, and a pharmaceutically acceptable carrier.
In a further embodiment, the present invention relates to a method of preventing transmission of malarial infection. The method comprising administering to a patient a recombinant virus of the present invention or Pfs25 protein purified from host cells of the present invention, in an amount sufficient to induce transmission blocking activity.